JSFiddle - React, Tailwind, and code Playground

HTML

<p>Select Month (1-Jan, ..., 12-dec)</p>

<select class="select_month">
  <option value="1">Jan</option>
  <option value="2">Feb</option>
  <option value="3">Mar</option>
  <option value="4">Apr</option>
  <option value="5">May</option>
  <option value="6">Jun</option>
  <option value="7">Jul</option>
  <option value="8">Aug</option>
  <option value="9">Sep</option>
  <option value="10">Oct</option>
  <option value="11">Nov</option>
  <option value="12">Dec</option>
</select>

JavaScript

$(".select_month").on('keyup',function(event){
    if(event.keyCode>=48&&event.keyCode<=57) {
        var index;
        var fentry;
        if($(this).hasClass('firstkey')) {
            index = 10+event.keyCode-48;
        } else {
            $(this).addClass('firstkey');
            index = event.keyCode-48;
        }
        $(".select_month option[value=" + index +"]").addClass("selectedmonth");
        setTimeout(function() {
            $(".selectedmonth").prop('selected', true);
            $('.firstkey').removeClass('firstkey');
            $('.selectedmonth').removeClass('selectedmonth');
        }, 500); 
    }
});